A crankcase lubricant is an oil used for general lubrication in an internal combustion engine where an oil sump is situated generally below the crankshaft of the engine and to which circulated oil returns. It is well known to include additives in crankcase lubricants for several purposes.
Environmental concerns have led to continued efforts to reduce the CO, hydrocarbon and nitrogen oxide (NOx) emissions of compression ignited (diesel-fuelled) and spark ignited (gasoline-fuelled) light duty internal combustion engines. Further, there have been continued efforts to reduce the particulate emissions of compression ignited light duty internal combustion engines. To meet the upcoming emission standards for passenger cars, original equipment manufacturers (OEMs) will rely on the use of additional exhaust gas after-treatment devices. Such exhaust gas after-treatment devices may include catalytic converters, which can contain one or more oxidation catalysts, NOx storage catalysts, and/or NH3 reduction catalysts; and/or a particulate trap.
Oxidation catalysts can become poisoned and rendered less effective by exposure to certain elements/compounds present in engine exhaust gasses, particularly by exposure to phosphorus and phosphorus compounds introduced into the exhaust gases by the degradation of phosphorus-containing lubricating oil additives. Reduction catalysts are sensitive to sulfur and sulfur compounds in the engine exhaust gases introduced by the degradation of both the base oil used to blend the lubricant, and sulfur-containing lubricating oil additives. Particulate traps can become blocked by metallic ash, which is a product of degraded metal-containing lubricating oil additives.
To insure a long service life, lubricating oil additives that exert a minimum negative impact on such after-treatment devices must be identified, and OEM specifications for “new service fill” and “first fill” lubricants typically require maximum sulfur levels of 0.30 mass %, maximum phosphorus levels of 0.08 mass %, and sulfated ash contents below 0.80 mass %; such lubricating oil compositions can be referred to as “low SAPS” (low sulfated ash, phosphorus, sulfur) lubricating oil compositions. In this respect, the European Automobile Manufacturers' Association (ACEA) C1-08 and C4-08 specifications impose even more stringent requirements and, for example, stipulate a sulphated ash content of less than or equal to 0.5 mass %; similarly, the Renault RN0720 specification stipulates a sulphated ash content of less than or equal to 0.50 mass %.
At the same time as complying with such low SAPS requirements, the lubricating oil composition, in use, must also provide adequate lubricant performance, including a permissible and defined level of thermal oxidative stability and viscosity increase due to thermal oxidation of the lubricant, in accordance with the particular specification. However, it has been found that reducing the amount of metal containing lubricant additives, for example metal containing detergents and metal containing anti-wear agents (e.g. ZDDP), in the lubricant typically has a negative impact on the thermal oxidative stability of the lubricant. Hence, low SAPS lubricating oil compositions, especially those having reduced sulphated ash levels, in use, tend to be more prone to thermal oxidation and may exhibit an unacceptably large increase in viscosity due to thermal oxidation of the lubricant. Although, it may be possible to improve the oxidative stability of such lubricants and counteract the thermally induced oxidative viscosity increase by including larger amounts of ashless (i.e. non-metal containing) antioxidants in the lubricating oil composition, such anti-oxidants are relatively expensive. There is therefore a need for a low sulphated ash, particularly low SAPS, lubricating oil composition which, in use, exhibits improved thermal oxidative stability and reduced levels of oil thickening due to thermal oxidation of the lubricant, without the need for the use of substantial amounts of relatively expensive ashless antioxidants.